Various radial designs for a turbine have been utilized since the dawn of the industrial revolution and their characteristic flow is found in numerous patents. The following qualitative graph is slightly modified from that found in “Mechanics of Fluids”; author: Irving Shames; ISBN 0-07-056385-3, page 616.
The graph illustrated in FIG. 1 shows through curves of efficiency versus relative velocity whether the optimal flow regime is either radial, axial or the transitional range between these two. At low speeds, the radial orientation provides superior efficiency. This graph is a qualitative selection tool. Calculating the specific speed of any particular application involves some computational complexity with dependent variables including, but not limited to, the fluid's physical properties, the equipment's rotational speed, and size of equipment. Without usage of any quantitative calculations, deductive reasoning can be used to categorize wind as having a very low specific speed. In the case of wind energy, a proposed site's wind speed distribution is important data for any type of economic feasibility study. Average wind speeds can be mapped into regions and given classification values that typically fall from one to five.
Axial flow wind turbines are typically built in class four regions. Class three regions are considered a minimum requirement for these types of wind turbines and class five or higher regions are not commonly encountered. At fifty meters height, a class four average wind speed falls between about 15.7 and 16.8 mph. Comparing this wind speed to that of typical propeller-driven air crafts at optimal cruising speeds illustrates the great disparity between the most efficient speed for axial flow wind devices and the operational speeds encountered in tower-mounted wind turbines.
Interestingly, in the operation of modern propeller-type wind turbines, both the cut-out speed (where blades are feathered to prevent over powering the generator) and the park speed (where rotation is stopped and one blade is aligned with the tower in a self-preservation mode) have relatively low specific speeds. Much work has been done upon the axial design using various airfoil profiles and blade rotation to achieve today's level of observed performance; however, the axial flow design is believed to utilize a much less than optimal flow orientation.